Each \(SU_k\)  can transmit data to the next hop in a hybrid conventional and backscatter manner. Each \(SU_k\)  can harvest energy just before its transmission time slot for conventional transmission. The transmission power of each \(SU_k\)  should be under a pre-determined threshold to prevent harmful interference to the PR. As depicted in Fig. 2, each time slot is divided into two sub-slots. The first sub-slot lasts \(\varphi _k\)  for conventional transmission, and the other lasts \(\tau _k - \varphi _k\)  for backscatter transmission. Each \(SU_k\)  has a super-capacitor to store harvested energy for conventional data transmission. Super-capacitors charge very rapidly; however, due to the high self-discharge rate, they discharge quite fast as well [1]}. Consequently, each \(SU_k\)  may spend its energy to transmit data right after the harvesting period. In this paper, we assume that the remaining energy in the super-capacitor or the energy harvested after the transmission cannot be used in the next time block due to the high self-discharge of the super-capacitor [2]}, [3]}. Based on this assumption, the stored energy of \(SU_k\)  is harvested solely during the interval \(\sum _{l=0}^{k-1}\tau _l, k = 1,...,K\) , and is equal to
[2]}
\(E_k = \zeta _k P_{t} h_k \sum _{i=0}^{k-1} \tau _i,\) 
